Although synthetic polymers are used extensively in society as plastics, rubbers, and textiles, the flammability of many polymers must be recognized as a safety hazard and an important challenge in polymer research. Flame retardants are incorporated into polymer materials as small molecule additives, or as part of the polymer backbone, to reduce flammability. A number of halogenated molecules, such as brominated aromatics, have been employed to reduce polymer flammability. Brominated aromatic flame retardants can be found in a wide-range of products, including computers, textiles, foam furniture, and construction materials. The environmental accumulation of halogenated flame retardants raises concerns that are restricting their use, and requires the development of nonhalogenated alternatives. In addition, some halogenated flame retardants release hydrogen halide gas upon combustion, which is especially undesirable in confined spaces, such as on aircrafts and ships. These concerns have led to an emphasis on nonhalogenated flame retardants in recent years. Some nonhalogenated flame retardant additives, such as alumina trihydrate, may compromise the physical and mechanical properties of polymers when loaded at high levels.
An ideal low-flammable polymer would be halogen-free and possess high thermal stability, low heat of combustion, and a low combustion heat release rate (HRR), with minimal release of toxic fumes. Intrinsically fire-resistant polymers that undergo significant carbonization upon heating are highly desirable, as carbonaceous char formation effectively averts combustion by producing an insulating layer on the polymer surface. Such char formation may also be realized from composite materials in which an additive ultimately provides the desired char.
The HRR of a material has been identified as a key characteristic of polymer flammability. Several calorimetry methods are available for measuring HRR during combustion, but these methods require relatively large sample quantities (˜100 g per experiment) and depend on several factors including ignition source, ventilation, sample thickness, orientation, and edge characteristics. Walters and Lyon developed pyrolysis combustion flow calorimetry (PCFC) as a method to evaluate polymer flammability on very small sample quantities (milligrams). PCFC measures the heat of combustion of the fuel gases that are released by the pyrolysis of a solid in an inert gas stream. The fuel gases then mix with excess oxygen and completely oxidize at high temperature. The instantaneous heat of combustion of the flowing gas stream is then measured by oxygen consumption calorimetry. The heat release capacity (HRC), defined as the maximum amount of heat released per unit mass per degree Kelvin (J/g K), is viewed as an inherent material property and a good predictor of flammability. (See, R. E. Lyon and R. N. Walters, J. Anal. Appl. Pyrolysis 2004, 71, 27.) HRC values obtained by PCFC, across a range of many polymer types, are found to scale with the larger, conventional benchscale flammability experiments.
Aromatic polyesters prepared from bisphenols and phthalic acids are important high performance engineering thermoplastics. Conventional bisphenol A (BPA)-based polyarylates are well-known and widely used, but exhibit higher-than-desired flammability (e.g., BPA-polyarylates have HRC ˜400 J/g K). Polyarylates containing 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene (bisphenol C, or BPC) are transparent and processable, and exhibit excellent mechanical and dielectric properties. BPC-based polymers are well within the “ultra fire-resistant” category (HRC<100 J/g K), with BPC-polyarylates and polycarbonates having reported HRC values of 21 and 29 J/g K, respectively, and high char yields (50-55%). However, the presence of halogen in BPC-based polymers, and the evolution of hydrogen chloride gas at elevated temperatures, remain concerns and may limit their adoption for scale-up and manufacturing as commodity materials.
BPC derivatives can be converted to the corresponding diphenylacetylene by loss of the chlorines, followed by phenyl migration. In BPC-containing polymers, this reaction represents a viable mechanism to char formation, in which the presence of chlorine sets up the rearrangement chemistry that leads to diphenylacetylene. In fact, diphenylacetylene-containing poly(aryl ether ketone)s showed heat release characteristics of similar magnitude to the corresponding BPC-versions. However, these alkyne-containing aromatic polymers are prone to side-reactions and crosslinking even at moderately high temperatures, and have less-than-optimal processibility and mechanical properties for many polymer materials applications. Accordingly, there is an ongoing search in the art for non-halogenated polymers or additives which promote charring and/or preclude combustion.
One approach has been the use of polyarylates incorporating a deoxybenzoin moiety, e.g., 4,4′-bishydroxydeoxybenzoin (BHDB), as a bisphenolic monomer. These polymers exhibited low combustion heat release rate and total heat of combustion, which is believed to arise from the thermally-induced conversion of BHDB to diphenylacetylene moieties that char by aromatization. See, K. A. Ellzey, T. Ranganathan, J. Zilberman, E. B. Coughlin, R. J. Farris, T. Emrick, Macromolecules 2006, 39, 3553. Pyrolysis combustion flow calorimetry (PCFC), an oxygen consumption technique for measuring heat release capacity (HRC), revealed exceptionally low HRC values for the BHDB-polyarylates (<100 J/g-K). (See, R. N. Walters, M. Smith, and M. R. Nyden, International SAMPE Symposium and Exhibition 2005, 50, 1118.) However, the rather low solubility of such polyarylate compounds limits their molecular weight and processibility. When bisphenol-A (BPA) and BHDB were used as co-bisphenols in the polyarylate synthesis, the solubility increased, but the flammability increased as well.